A new study finds that friends share more genes than strangers do.

If you think your friends are like family, you may be more right than you know.

A new study finds that friends share more genes than strangers do — not enough to make them like brothers and sisters, but enough to make them something like fourth cousins.

"Birds of a feather flock together. We all know this is true at a behavioral level, so we wanted to know if it extended all the way to your DNA," says study co-author James Fowler, a professor of medical genetics and political science at the University of California-San Diego.

It does, according to the study, published Monday in the Proceedings of the National Academy of Sciences. The study involved 1,932 individuals tested in pairs of unrelated friends and strangers. Another recent study, by other researchers, found more genetic similarities in spouses than in strangers.

An obvious explanation might be that friends and lovers of the same racial, ethnic and geographic backgrounds find one another most easily, with the least cultural friction — meaning any matching genes are just along for the ride. But Fowler and co-author Nicholas Christakis, professor of sociology, evolutionary biology and medicine at Yale University, say they don't think that's the best explanation for their findings — partly because almost all of their subjects were of European descent and all had roots in the same town, Framingham, Mass.

The researchers took advantage of the fact that several generations of Framingham residents and descendants contributed not only blood samples but information about their lives — including their friendships — for an ongoing series of health studies. Finding that friends are more genetically alike than strangers in that not-so-diverse population suggests something more is going on, they say.

They say it is possible that people actively choose friends like themselves, but also that similar people are drawn to similar environments or placed in them by others (employers, parents and schools, for instance).

One intriguing possibility: that the sense of smell may draw similar people together. The researchers found genes controlling smell were among those most likely to match.

So maybe people who share a love for the smell of coffee end up meeting at Starbucks, Fowler says, while "someone who hates the smell of coffee never walks in the door."

That's just speculation, says Benjamin Domingue, a research associate at the Institute of Behavioral Science at the University of Colorado-Boulder. He co-wrote the recent paper finding genetic similarities among spouses but was not involved in the new study. He says Fowler and Christakis make a good case that the similarities among friends are real — and even show they can predict roughly how likely it is two people will be friends by looking at their genes and assigning a "friendship score." That's "impressive," Domingue says. It would be interesting to see whether the findings hold up in a larger, more diverse population, he says.

The researchers also can only speculate about why evolution might favor friends with more genes in common. When friends help friends, perhaps more survive to pass those genes along, they write: "If one builds a fire because he feels cold in the same circumstances as the other, both benefit."